If you like to follow all the things people like to argue about on the internet, you know there's a huge debate going on now over whether it is possible to have a vehicle, powered only by the wind, outrun the wind. Most people seem to intuitively reject the concept right off the bat: it seems a bit like some kind of crackpot perpetual motion machine, especially if you hear that it is some sort of windmill that powers the wheels, or the wheels are turning a propeller, or something crazy like that. What's next, a solar powered light bulb?

A "downwind faster than the wind" vehicle can most certainly work, and you don't need a physics degree to understand why. Although others have attempted to explain it, obviously not everyone is convinced. So I will try explaining it in my own way, possibly attacking the "why this is tricky to wrap one's head around" issue as much as the problem itself (sorry, it's a bit of a habit of mine).

I see two issues that make a DWFTTW vehicle hard to visualize and understand. Independently, neither is so difficult, but when put together they can give our brains too much to juggle at once.

1. Slippage -- devices that involve fluid dynamics are inherently harder to completely understand than "positive drive" systems -- such as those exclusively using gears, non-slipping wheels, belts, pulleys and the like -- since there will always be a hard-to-establish amount of slippage. This makes it difficult to calculate or "draw it up" solely on paper, being a much more complex-to-evaluate aerodynamics problem rather than something that can be worked out using geometry alone.
2. "Feedback loop" mechanisms -- some devices can be inherently unintuitive and hard to visualize and explain due to having power applied in less-than-straightforward ways. The line between "driving" and "driven" (e.g. motor vs. generator, propeller vs. windmill, pump vs. turbine, etc) can become very blurry and the systems might be best understood as "finding an equilibrium," which is a lot more difficult to grasp than a straightforward "A pushes B which turns C" sort of thing. They also may trigger a reflexive -- but in this case, undeserved -- "that's a perpetual motion machine" reaction.

To help visualize the problem, let's compare our debated vehicle to three that are easier to comprehend, each of which lacks one, the other, or both of the complicating issues above.

Welcome to Karmatics. I've decided to go ahead and start a site/blog to feature some of my stuff. There's a lot more than what you see here now, but for starters there is Aardvark (something I made in 2005 and has been a big hit, and much copied), the brand new Chameleon (a little freebie that started as a proof of concept of some of the neat things you can do by modifying the transparency of images with javascript), and finally the yet-to-be-named "change color in photo" thingamagig, which is still just a proof of concept, but something that amazed even me that it works...